1. Field of the Invention
The present invention relates to dust collecting devices for use with the machining apparatus, such as table circular-saws for cutting workpieces, e.g., wood, so that cutting chips produced during the cutting operation can be collected by the collecting devices.
2. Description of the Related Art
Known table circular saws have a table and a saw unit. A workpiece can be placed on the table. The saw unit is vertically pivotally supported on the table and has an electric motor and a circular saw blade that is rotatably driven by the motor. Therefore, as an operator pivots the saw unit downward, the rotating circular saw may cut the workpiece placed on the table.
Some known table circular saws have dust collecting devices for collecting the cutting chips produced during the cutting operation to favorably maintain the surrounding working environment. Known dust collecting devices have a simple construction including a dust bag connected to a dust discharge pipe disposed at the rear portion of a blade case of a saw unit of a table circular saw. The blade case covers substantially the upper half of a saw blade. Therefore, as the saw blade rotates during the cutting operation, a flow of air is produced within the blade case, so that the air flows into the dust bag together with the cutting chips to the result that the cutting chips are collected by the dust bag. However, this type of dust collecting device has a problem that the collected dust may flow back toward the saw unit, because the dust bag as well as the dust discharge pipe moves (or inclines) as the saw unit vertically pivots.
In order to solve this problem, Japanese Laid-Open Patent Publication No. 2005-96233 has proposed to provide a back-flow preventing valve disposed within a dust discharge pipe of a saw unit. The valve can be opened by the force of flow of air produced by the rotating saw blade, so that dust can be collected by the dust bag. On the other hand, the valve can be closed by its gravity force or by a suitable biasing device when the rotation of the saw blade has been stopped, so that the collected dust can be prevented from flowing back toward the saw unit.
However, in the case of the dust collecting device of this publication, if the force of flow of air (i.e., an air-blowing force) produce by the rotating saw blade is not sufficient, the valve may not be opened enough. In such a case, the cutting chips may not be successfully moved into the dust bag to result lower dust collecting efficiency.
Therefore, there has been a need for a dust collecting device that can stably and effectively collect dust.